When you are connected to a mail exchanger, you CANNOT USE the backspace (delete) key. So any mistakes you make mean you must reconnect or send it as it is. Once you know what's going on, it may be worth setting up a telnet macro to do the entire process for you. That's assuming you'll be doing this a lot.
For this example, I will be emailing poor_sod@gullible.com, and the email will appear to come from nastygeezer@brutal.com. You can in fact email anybody with this method - the way the internet works means that (almost) anyone with any email address can be fakemailed.
Every computer you connect to to use this method will look slightly different, but the commands are generic, and should work anywhere.
And finally - these instructions are written for a Unix user - the method is basically the same for a Windoze user - but if you can't figure out what to change, you probably shouldn't be doing this at all. Some Windoze users don't have NSLookup, if not, you can try IntNet's online NSLookup.

Step 1 - Getting mail exchanger data and connecting
You must use a program called "nslookup" to look up information about the computer where poor_sod has his email address. In this case, gullible.com. On a standard *nix system, this can be achieved like this:

An nslookup will produce quite a lot of data, but the only thing you are looking for is a "mail exchanger" - there are often several, but any of them will do. At this point, we must connect to the mail exchanger using port 25 - this is achieved like this:

What we must do now is make the email look as if it came from a proper email program, rather than us just handtyping it in - this is important. If we don't put in credible data, most email clients will reject it. What we must do is add the headers, and then the email.

Code:

DATA
354 Ready for data - end input with a "." on a new line
Date: 4/7/108
Time: 1:05:30 (GMT+300)
From: Mr Fake Guy <nastygeezer@brutal.com>
To: Poor Sod <poor_sod@gullible.com>
Subject: I'm a nasty geezer.
The email goes here. We've typed the date and time (exactly! look!)
and then the names and email addresses of the sender and the recipient,
and the subject. Then just leave a blank line, and type the email!
When we want to end, we just do the full stop (period) on a new line
as the message said...
.
220 OK (ESMTP id 20023/197A4BB)
QUIT
Connection closed by foreign host
%

Hope this helped you if you have any questions please don't hesitate to ask me